Area Forecast Discussion
Issued by NWS Des Moines, IA

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297
FXUS63 KDMX 152219
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
419 PM CST Sun Jan 15 2017

.SHORT TERM.../Tonight through Monday/
Issued at 418 PM CST Sun Jan 15 2017

The impending winter storm remains the primary concern but
confidence is low with many moving parts. Temperatures from the
surface to the 1-1.5km warm layer are changing with time and often
passing through critical values 2-3C. How this impacts the surface
and roads is questionable too with current road temps mainly above
freezing and the degree of nocturnal cooling uncertain due to
increasing and lowering clouds and precip starting. Latent heat
release from convection and surface freezing will also cause
concerns. Confidence in precip as a whole is medium to high however
with the models in fairly good agreement as the system progresses.

The current scenario has broad mid and upper level lift between the
kinematic forcing ahead of the potent short wave/PV anomaly
approaching MN/TX and the right entrance region of the Great Lakes
jet segment. Weak warm/theta-e advection is also making
contributions into the MO Valley resulting in precip spreading
through KS/NE, and soon into southern and western IA. Forecasting
precip type has been problematic today due to trying to account for
evaporative cooling through the 1-1.5km warm but dry layer. With
temps aloft 3C+, melting has occurred resulting in mainly freezing
rain reports, but eventually this will crash toward the wet bulb
likely mixing in more frozen precip. Into the evening the forcing
will remain similar but push farther into Iowa with similar warm/dry
layer uncertainties reducing confidence in sensible weather details,
not to mention road temp questions.

The thermodynamic and kinematic forcing all increases into Monday
morning just ahead of the upper system and should increase precip
rates until diminishing somewhat through the afternoon all but north
as the dry slot rotates. This includes lower lapse rates and thunder
potential southeast ahead of the dry slot. With waves of precip
upstream from the immediate MO Valley all the way toward the upper
low, precip amounts have become more disconcerting with quarter inch
plus ice accums now expanding farther north and east. With higher
confidence in the colder air farther north have expanded the Ice
Storm Warning somewhat along the I80 and Highway 30 corridors. Some
ice accums are now up to 0.4.  Surface temps and dewpoints suggest
the rain/freezing rain line should roughly be SW-NE near a Carroll-
Waterloo line but ice accums may continue south the cold air,
especially on elevated untreated surfaces which have already
accumulated ice through the night and into the morning. The
aforementioned latent heat concerns will also be coming more into
play about this time as well with reduced confidence.

.LONG TERM.../Monday night through Sunday/
Issued at 418 PM CST Sun Jan 15 2017

The winter storm system affecting our area tonight and Monday will
begin to move off to the northeast and east on Monday night.
During the evening and early nighttime hours precipitation is
still expected, with forecast soundings indicating snow or sleet
northwest and mainly freezing rain for a big swath across central
Iowa. QPF will be diminishing by Monday night but additional ice
accumulations of around 0.05-0.10 inches are still likely in some
areas. Late Monday night into Tuesday morning and forcing will
diminish markedly as the low pressure center moves away from our
area, however forecast soundings indicate persistent low-level
moisture and we may see some lingering freezing drizzle. Have
maintained 20 to 40 POPs during this time with light ice mention,
but little to no additional accumulation.

From late Tuesday through at least Wednesday night the forecast is
dry with steadily moderating temperatures as a modest deep-layer
ridge builds over the region. By Wednesday afternoon this will
result in temperatures climbing into the mid 30s-north to mid-40s
south, melting most or all remaining ice from the early week
system. From Thursday through Friday a week mid/upper level storm
system will move up the high plains just to our west and may
produce some light precipitation across Iowa. However, the forcing
is unimpressive and moisture is limited with temperatures expected
to remain near or above freezing, so this should manifest as
mainly light rain, possibly with a light wintry mix north on
Friday morning.

The Thursday-Friday system will be the first in a series of
vorticity maxima carving out and rounding the base of a deepening
western U.S. trough by next weekend and into early next week. The
result will be a continuation of periodic precipitation chances
through the extended forecast period. Temperatures should remain
fairly warm beneath the broad southwesterly flow aloft so chances
for winter precipitation will be somewhat limited, at least until
the trough axis moves through sometime beyond the current 7-day
forecast period.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1149 AM CST Sun Jan 15 2017

Conditions will deteriorate across most elements into the night.
VFR conditions and a light wintry mix are already underway south
and this precip is expected to slowly spread north through the
state later tonight. This will also eventually bring lower
ceilings and visibility as low level moisture increases with a
similar south to north progression of IFR/LIFR conditions, which
should last through the valid period once they begin.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Ice Storm Warning from 9 PM this evening to midnight CST Monday
night for IAZ023-033>038-044>050-057>060.

Ice Storm Warning from 9 PM this evening to 6 PM CST Monday for
IAZ061-062.

Ice Storm Warning until 6 PM CST Monday for IAZ070>073-081>084-
092>095.

Winter Weather Advisory until 6 PM CST Monday for IAZ074-075-085-
086-096-097.

Winter Weather Advisory from midnight tonight to midnight CST
Monday night for IAZ004>007-015>017.

Winter Weather Advisory from 9 PM this evening to midnight CST
Monday night for IAZ024>028-039.

&&

$$

SHORT TERM...Small
LONG TERM...Lee
AVIATION...Small



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